I'd say about 10% of my monthly income goes to gear on average but obviously the last few months have been much much more since starting Eurorack. Plus I have a lot of non average purchases throughout a year that increases that average substantially... so an average is probably closer to 20% or so.
Right now since getting into modular I've done a ton of gear flipping with rack units, high end effects, guitars, etc. probably pulled in $20k to $25k off selling gear since going modular so that funded a lot of this. I don't flip often but when I do it's in huge sports like that. I actually used to make more money and because of that I have a ton of gear that is basically like having a savings account I can play. When I need to I sell off large chunks and use that cash to buy more gear.
Also, just preordered this guy!!